Bump python_requires >= 3.8.

Code to handle CancelledError (as subclass of Exception) was removed.

Author: moodyjon

setup.py

@@ -18,7 +18,7 @@
     long_description_content_type="text/markdown",
     keywords="lbry protocol media",
     license='MIT',
-    python_requires='>=3.7',
+    python_requires='>=3.8',
     packages=find_packages(exclude=('tests',)),
     zip_safe=False,
     entry_points={